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Isthminia panamensis, a new fossil inioid (Mammalia, Cetacea) from the Chagres Formation of Panama and the evolution of 'river dolphins' in the Americas.

Pyenson ND, Vélez-Juarbe J, Gutstein CS, Little H, Vigil D, O'Dea A - PeerJ (2015)

Bottom Line: Phylogenetic analysis of fossil and living inioids, including new codings for Ischyrorhynchus, an enigmatic taxon from the late Miocene of Argentina, places Isthminia as the sister taxon to Inia, in a broader clade that includes Ischyrorhynchus and Meherrinia, a North American fossil inioid.This phylogenetic hypothesis complicates the possible scenarios for the freshwater invasion of the Amazon River system by stem relatives of Inia, but it remains consistent with a broader marine ancestry for Inioidea.Based on the fossil record of this group, along with Isthminia, we propose that a marine ancestor of Inia invaded Amazonia during late Miocene eustatic sea-level highs.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Paleobiology, National Museum of Natural History, Smithsonian Institution , Washington, DC , USA ; Departments of Mammalogy and Paleontology, Burke Museum of Natural History and Culture , Seattle, WA , USA.

ABSTRACT
In contrast to dominant mode of ecological transition in the evolution of marine mammals, different lineages of toothed whales (Odontoceti) have repeatedly invaded freshwater ecosystems during the Cenozoic era. The so-called 'river dolphins' are now recognized as independent lineages that converged on similar morphological specializations (e.g., longirostry). In South America, the two endemic 'river dolphin' lineages form a clade (Inioidea), with closely related fossil inioids from marine rock units in the South Pacific and North Atlantic oceans. Here we describe a new genus and species of fossil inioid, Isthminia panamensis, gen. et sp. nov. from the late Miocene of Panama. The type and only known specimen consists of a partial skull, mandibles, isolated teeth, a right scapula, and carpal elements recovered from the Piña Facies of the Chagres Formation, along the Caribbean coast of Panama. Sedimentological and associated fauna from the Piña Facies point to fully marine conditions with high planktonic productivity about 6.1-5.8 million years ago (Messinian), pre-dating the final closure of the Isthmus of Panama. Along with ecomorphological data, we propose that Isthminia was primarily a marine inhabitant, similar to modern oceanic delphinoids. Phylogenetic analysis of fossil and living inioids, including new codings for Ischyrorhynchus, an enigmatic taxon from the late Miocene of Argentina, places Isthminia as the sister taxon to Inia, in a broader clade that includes Ischyrorhynchus and Meherrinia, a North American fossil inioid. This phylogenetic hypothesis complicates the possible scenarios for the freshwater invasion of the Amazon River system by stem relatives of Inia, but it remains consistent with a broader marine ancestry for Inioidea. Based on the fossil record of this group, along with Isthminia, we propose that a marine ancestor of Inia invaded Amazonia during late Miocene eustatic sea-level highs.

No MeSH data available.


Related in: MedlinePlus

Close-up of upper and lower dentition.The dentition of Isthminia panamensis (USNM 546125) in close view. (A–E) Upper dentition including the rostrum (A) and isolated teeth collected near the skull at the outcrop surface, showing (B), an upper left posterior tooth (likely PC3) and (C), an upper left posterior tooth. (F–I) Lower dentition including the mandible (F, G), shown in two parts, with overlapping images over the mandibular symphysis. (H–I) An additional isolated left tooth posterior (almost certainly PC12) was collected at the type locality. Dashed lines with arrowheads indicate alignment for the occlusion of upper and lower dentition.
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fig-10: Close-up of upper and lower dentition.The dentition of Isthminia panamensis (USNM 546125) in close view. (A–E) Upper dentition including the rostrum (A) and isolated teeth collected near the skull at the outcrop surface, showing (B), an upper left posterior tooth (likely PC3) and (C), an upper left posterior tooth. (F–I) Lower dentition including the mandible (F, G), shown in two parts, with overlapping images over the mandibular symphysis. (H–I) An additional isolated left tooth posterior (almost certainly PC12) was collected at the type locality. Dashed lines with arrowheads indicate alignment for the occlusion of upper and lower dentition.

Mentions: Both right and left mandibles are preserved intact and remain articulated via an osseous symphyseal articulation (Figs. 8 and 9; Class IV jaw joint of Scapino, 1981). The length of the mandibular symphysis (21.0 cm) is approximately 43% of the entire length of the mandible. The mandibles possess nearly all of the original lower teeth; the lower first incisors are missing, along with posterior most three teeth of the right mandible (although one isolated tooth is a perfect fit for PC12; see Fig. 10). Both the right and left mandibles possessed 18 and 17 lower teeth, respectively, although the degree of bone remodeling posterior of left PC13 leads us to presume that 18 teeth is the likely maximum lower tooth count (Fig. 10). Posterior margins are incomplete for both sides of the mandible, and the left angular process appears intact and there is a weak suggestion of the osteological structure where the left articular condyle would have been. The right articular condyle is missing. Most of the mandibles are well preserved, although much of the right acoustic window is degraded from erosion and/or diagenesis (Fig. 9).


Isthminia panamensis, a new fossil inioid (Mammalia, Cetacea) from the Chagres Formation of Panama and the evolution of 'river dolphins' in the Americas.

Pyenson ND, Vélez-Juarbe J, Gutstein CS, Little H, Vigil D, O'Dea A - PeerJ (2015)

Close-up of upper and lower dentition.The dentition of Isthminia panamensis (USNM 546125) in close view. (A–E) Upper dentition including the rostrum (A) and isolated teeth collected near the skull at the outcrop surface, showing (B), an upper left posterior tooth (likely PC3) and (C), an upper left posterior tooth. (F–I) Lower dentition including the mandible (F, G), shown in two parts, with overlapping images over the mandibular symphysis. (H–I) An additional isolated left tooth posterior (almost certainly PC12) was collected at the type locality. Dashed lines with arrowheads indicate alignment for the occlusion of upper and lower dentition.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4562255&req=5

fig-10: Close-up of upper and lower dentition.The dentition of Isthminia panamensis (USNM 546125) in close view. (A–E) Upper dentition including the rostrum (A) and isolated teeth collected near the skull at the outcrop surface, showing (B), an upper left posterior tooth (likely PC3) and (C), an upper left posterior tooth. (F–I) Lower dentition including the mandible (F, G), shown in two parts, with overlapping images over the mandibular symphysis. (H–I) An additional isolated left tooth posterior (almost certainly PC12) was collected at the type locality. Dashed lines with arrowheads indicate alignment for the occlusion of upper and lower dentition.
Mentions: Both right and left mandibles are preserved intact and remain articulated via an osseous symphyseal articulation (Figs. 8 and 9; Class IV jaw joint of Scapino, 1981). The length of the mandibular symphysis (21.0 cm) is approximately 43% of the entire length of the mandible. The mandibles possess nearly all of the original lower teeth; the lower first incisors are missing, along with posterior most three teeth of the right mandible (although one isolated tooth is a perfect fit for PC12; see Fig. 10). Both the right and left mandibles possessed 18 and 17 lower teeth, respectively, although the degree of bone remodeling posterior of left PC13 leads us to presume that 18 teeth is the likely maximum lower tooth count (Fig. 10). Posterior margins are incomplete for both sides of the mandible, and the left angular process appears intact and there is a weak suggestion of the osteological structure where the left articular condyle would have been. The right articular condyle is missing. Most of the mandibles are well preserved, although much of the right acoustic window is degraded from erosion and/or diagenesis (Fig. 9).

Bottom Line: Phylogenetic analysis of fossil and living inioids, including new codings for Ischyrorhynchus, an enigmatic taxon from the late Miocene of Argentina, places Isthminia as the sister taxon to Inia, in a broader clade that includes Ischyrorhynchus and Meherrinia, a North American fossil inioid.This phylogenetic hypothesis complicates the possible scenarios for the freshwater invasion of the Amazon River system by stem relatives of Inia, but it remains consistent with a broader marine ancestry for Inioidea.Based on the fossil record of this group, along with Isthminia, we propose that a marine ancestor of Inia invaded Amazonia during late Miocene eustatic sea-level highs.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Paleobiology, National Museum of Natural History, Smithsonian Institution , Washington, DC , USA ; Departments of Mammalogy and Paleontology, Burke Museum of Natural History and Culture , Seattle, WA , USA.

ABSTRACT
In contrast to dominant mode of ecological transition in the evolution of marine mammals, different lineages of toothed whales (Odontoceti) have repeatedly invaded freshwater ecosystems during the Cenozoic era. The so-called 'river dolphins' are now recognized as independent lineages that converged on similar morphological specializations (e.g., longirostry). In South America, the two endemic 'river dolphin' lineages form a clade (Inioidea), with closely related fossil inioids from marine rock units in the South Pacific and North Atlantic oceans. Here we describe a new genus and species of fossil inioid, Isthminia panamensis, gen. et sp. nov. from the late Miocene of Panama. The type and only known specimen consists of a partial skull, mandibles, isolated teeth, a right scapula, and carpal elements recovered from the Piña Facies of the Chagres Formation, along the Caribbean coast of Panama. Sedimentological and associated fauna from the Piña Facies point to fully marine conditions with high planktonic productivity about 6.1-5.8 million years ago (Messinian), pre-dating the final closure of the Isthmus of Panama. Along with ecomorphological data, we propose that Isthminia was primarily a marine inhabitant, similar to modern oceanic delphinoids. Phylogenetic analysis of fossil and living inioids, including new codings for Ischyrorhynchus, an enigmatic taxon from the late Miocene of Argentina, places Isthminia as the sister taxon to Inia, in a broader clade that includes Ischyrorhynchus and Meherrinia, a North American fossil inioid. This phylogenetic hypothesis complicates the possible scenarios for the freshwater invasion of the Amazon River system by stem relatives of Inia, but it remains consistent with a broader marine ancestry for Inioidea. Based on the fossil record of this group, along with Isthminia, we propose that a marine ancestor of Inia invaded Amazonia during late Miocene eustatic sea-level highs.

No MeSH data available.


Related in: MedlinePlus